The actual merging process will take several years, probably 2-3. AT&T isn't stupid, they don't want T-Mobile customers to churn en masse.

2-3 years, NO WAY! You think ATT would say their 4G roll out will take another 2-3 years, nope! They've already said their 4G roll out is dependent on this buy out.
And where would TMO customers go, Sprint? They won't have much of a choice. The FCC & DOJ can put on a front that they're going to scrutinize this buy out but the fact is the Government has said they want to bring broadband internet to everyone and wireless is the way to do that. I think it'll happen quicker than you think.